Need Clarification on Proper Rear End Gear Oil

[Papa]

My Moser 3-link rear came with a couple of sheets of instructions for break-in and brake installation advice, but nothing on what oil I should use. The FF assembly manual lists 80W90 with 4 oz. of friction modifier. Moser's web site FAQ lists different fluids based on the specific assembly (Eaton, etc.), which I'm embarrassed to admit that I have no idea what is in the housing. Here is a picture:

http://i86.photobucket.com/albums/k99/dcabral99/Factory%20Five/20170530_101154_zpsdmxsv2bq.jpg (http://s86.photobucket.com/user/dcabral99/media/Factory%20Five/20170530_101154_zpsdmxsv2bq.jpg.html)

As always, I'm at the mercy of the experts and appreciate your advice.

[Gordon Levy]

use 75/90 gear oil and I use 6 or so oz of friction modifier.

[boat737]

My Moser M9 (Ford 9") 3 link has an Eaton Detroit TruTrak differential. My Eaton instructions say not to use a friction modifier with that differential. The recommended rear end grease for mine is Motul Gear 300 75W90 synthetic, 317811 / 100118. Per Moser, the Motul Synthetic has no friction modifiers like most synthetics do.

[Gordon Levy]

That pic is not an eaton

[CraigS]

That is a standard Ford TracLok (SP?). I use Mobil1 80-90 and no modifier.

[rich grsc]

Only one correct answer.

[DaveS53]

From what I've read, the current recommendation is 75W-140. If you have a posi unit with clutches, it should require limited slip additive, or fluid with the LS additive already in it. A truetrac helical gear posi has no clutches and does not require a LS fluid.

I haven't read anything yet that rules out synthetic fluid with a truetrac. Lube is discussed starting on page 24.

file:///C:/Users/Dave/Downloads/Eaton_Differentials_Owners_Manual_0314_LR%20(2).PD F

[Bob Cowan]

use 75/90 gear oil and I use 6 or so oz of friction modifier.


Only one correct answer.

And this is it. :) You need a "heavy" oil if you have a have heavy vehicle and/or you're putting it to heavy duty use. In a light weight vehicle and standard use, a heavy weight oil will not perform as well, and cost you more power to move around. Unless you're racing, stay with the 75/90 and don't worry about it.

[Papa]

Thanks for clearing up my confusion everyone. :confused: So after reading everything above and doing some more searching, I'm at least convinced that I have the Ford Trac-Lok. I also see that 75W90 seems to be the predominate recommendation. Should I use standard or synthetic? Any preferred brands for the oil and LS additive?

I'm going to be using this car exclusively on the road, no racing.

[Gordon Levy]

Synthetic is fine but not necessary. Pretty much any friction modifier will work.